1. Field of the Invention
The present invention relates to a sheet width aligning device for aligning the width of a sheet as transported, and a sheet transport device provided with the sheet width aligning device and also relates to an image forming apparatus provided with the sheet transport device.
2. Description of the Related Art
In recent years, image forming apparatuses such as copiers capable of duplex printing sheets have been widely used. In such image forming apparatuses, the transport destination of a sheet after being subjected to a fixing process is switched to a discharge tray or a reversing tray according to whether simplex printing or duplex printing is to be performed.
In the case of simplex printing, a switching guide is set to a discharge tray side to discharge a sheet finished with the fixing process to the outside. On the other hand, in the case of duplex printing, the switching guide is set to a reversing tray side to introduce a sheet finished with the fixing process on one side to the reversing tray. Thereafter, the sheet is returned to an image forming unit to have an image transferred to the other side thereof after being reversed upside down. The sheet finished with duplex printing is discharged to the outside via the switching guide set to the discharge tray.
Upon duplex printing, printing positions on one and the other sides of a sheet need to be aligned. Japanese Unexamined Patent Publication No. H08-152772 (published on Jun. 11, 1996) discloses a technique for aligning printing positions on one and the other sides of a sheet by moving a focusing lens in a direction orthogonal to an optical axis according to a displacement amount of the lateral registration of a sheet tray storing sheets and moving first and second jogger fences for performing the width alignment of the sheet based on the size of sheets stored in the sheet tray and a lateral registration adjustment value peculiar to the sheet tray.
In the case of duplex printing, width alignment is performed after printing on one side so that printed positions are not displaced on the one and other sides due to the oblique transportation of a sheet upon printing on the other side. This width alignment is performed by regulating the positions of a sheet in a widthwise direction according to the sheet size by a pair of cursors (corresponding to the first and second jogger fences in Japanese Unexamined Patent Publication No. H08-152772). However, even sheets of the same size may have different widths due to the contraction of sheets caused by a fixing process using a heating roller and a pressure roller, size errors and cutting errors of sheets of the respective sheet manufacturers. In such a case, the position of the sheet cannot be perfectly adjusted when adopting the method of simply moving the above cursors according to the standard size (A4, B5, etc.) of the sheet for width alignment, which has caused pattern misalignment on one and other sides.
According to the image forming apparatus of Japanese Unexamined Patent Publication No. H08-152772, the width alignment is performed based on the sheet size and the lateral registration adjustment value corresponding to each sheet tray, it is not possible to precisely align the width of each sheet when differences in sheet width exist among sheets of the same size, and it is therefore not possible to prevent the misalignment in respective printing positions on one and the other sides.